Conventionally, various techniques have been developed, as follows. A raw material gas obtained by adding a nitrogen gas of several thousands ppm or more to an oxygen gas is supplied to an ozone generator to generate a high concentration ozone gas, and this high concentration ozone gas is often used in an ozone treatment process such as formation of an ozone oxide insulating film and ozone washing in the field of semiconductor fabrication. In the field of semiconductor fabrication and the like, in a case of supplying an ozone gas to a multiple ozone treatment apparatus including a plurality of ozone treatment apparatuses, it is generally conceivable to build an ozone gas supply system in which a plurality of ozone generation mechanisms (means) each including an ozone generator, an ozone power source, a flow rate controller (MFC), and the like, are provided corresponding to the plurality of ozone treatment apparatuses, respectively, so that the ozone generation mechanisms independently supply an ozone gas to the corresponding ozone treatment apparatuses.
Conventionally, as shown in FIG. 26, in order that an ozone generator 71 that includes electrodes 71a and 71b, a dielectric material 71c, and the like, and that is supplied with power from an ozone power source 72 can generate an ozone gas with an increased efficiency, an ordinary oxygen gas contains a nitrogen gas of about 50 to several thousands ppm, and in a case of a high purity oxygen gas containing less nitrogen (less than 50 ppm), a small amount of a N2 gas (500 ppm or more) as well as the high purity oxygen gas is added to the interior of the ozone generator.
Accordingly, in a case of a N2 gas of 500 ppm or more is contained in a raw material oxygen gas, high concentration ozone is generated due to a catalytic reaction of a small amount of NO2 that is generated as a result of a discharge reaction shown in FIG. 27. Particularly, adding a nitrogen gas of 500 to 20000 ppm allows ozone to be generated efficiently due to a catalytic reaction of a small amount of nitrogen dioxide that is generated as a result of the discharge. This consequently generates ozone having the highest concentration. It has been verified from experiments that a raw material gas in which the amount of added nitrogen is 500 to 20000 ppm is the optimal condition in terms of ozone generation performance.
As shown in (1) to (3) below, the discharge reaction shown in FIG. 27 achieves generation of high concentration ozone by using raw material oxygen O2, photoelectric discharge light, and a catalytic gas of a small amount of NO2.
(1) Reaction for generating a small amount of NO2 gas by discharge                Reaction for ionizing nitrogen moleculeN2+e→2N+        Reaction for generating NO2 2N++O2+M→NO2                     (generate a NO2 gas of several ppm to several tens ppm)                        
(2) Generation of an oxygen atom O by a catalytic effect of NO2 caused by discharge light                Photodissociation reaction of NO2 NO2+hν→NO+O         Oxidation reaction of NONO+O2 (raw material oxygen)→NO2+O        
* In these two reactions, NO2 acts as a catalyst to generate an oxygen atom.
A large amount of oxygen atoms O generated by the reaction (2) react with oxygen gas molecules O2, to generate ozone O3.
(3) Generation of ozone O3 (three-body collision)R2;O+O2+M→O3+M
Through (1) to (3) described above, high concentration ozone is generated.
However, when a large amount of N2 gas is contained in the raw material oxygen gas, not only an ozone gas but also NOx by-product gases, such as a N2O5 gas and N2O gas, and nitric acid, are generated as a result of a silent discharge in the ozone generator. Specific formulas of generation of the NOx by-product gases and nitric acid are as follows.N2+e→N2*+e→N2+hν(310,316,337,358 nm)
N2*; excitation of nitrogen
ultraviolet light by nitrogen gasH2O+e→H+OH+e (dissociation of water vapor)N2+e→2N−+e (dissociation of nitrogen molecule)NO2+hν(295 to 400 nm)→NO+O(3P)H+O2+M→HO2+MHO2+NO→OH+NO2 N2O5±H2O→2HNO3 OH+NO2+M→HNO3+M
In this manner, not only the ozone gas but also NOX by-product gases and nitric acid are generated.
When a large amount of NOX by-product is generated, a NOX gas component reacts with moisture contained in the raw material gas, to generate a nitric acid (HNO3) cluster (vapor). Thus, an ozonized gas is extracted under a state where a small amount of NOX gas and nitric acid cluster, together with oxygen and the ozone gas, are mixed. When the small amount of nitric acid cluster is several hundred ppm or more, the following problems are caused. That is, rust of chromium oxide or the like resulting from nitric acid is deposited on an inner surface of a stainless pipe that is an ozone gas outlet pipe. As a result, a metal impurity is mixed into a clean ozone gas. In a reaction gas for a semiconductor manufacturing apparatus, the metal impurity adversely affects fabrication of a semiconductor. Additionally, the small amount of nitric acid cluster thus generated acts as a reaction poisonous substance and adversely affects “an etching process on a silicon oxide film by using ozone” and “washing of a wafer or the like by using ozone water”, which are performed in the semiconductor manufacturing apparatus.
In a ozone gas supply system including an ozone generator, an ozone power source, and the like, it is generally conceivable that an ozone generator, an ozone power source, a raw material gas pipe system line, an output gas pipe system line, and the like, are provided, and the number of each of them is equal to the number of system lines included in a multiple ozone treatment apparatus. The raw material gas pipe system line supplies an ozone gas or a raw material gas to the ozone generator via a flow rate adjuster such as an MFC for controlling the flow rate of the ozone gas or the raw material gas. The output gas pipe system line includes an ozone concentration detector and an ozone flow meter. The ozone concentration detector has pressure adjuster, such as an APC, for controlling gas atmosphere pressure in the ozone generator. The ozone concentration detector detects a concentration of the ozone gas outputted from the ozone generator.
However, it is impossible to supply high concentration ozonized oxygen with a very small amount of NOX by-product though the amount of NOX is large. Additionally, a very large space is required for building an ozone generation system compatible with such a multiple ozone treatment apparatus, and moreover, a still larger system configuration is required for building a system that supplies an ozone gas while coordinately controlling the multiple ozone treatment apparatus. Thus, there are problems of costs, an installation space, and the like, which causes many disadvantages in a practical use.
Accordingly, an attempt was made to generate ozone by using only a high purity oxygen gas without any nitrogen gas being contained in the conventional ozone generator. As a result, however, only a small amount ozone was generated. The reason therefor is considered as follows. The oxygen molecule, which constitutes the raw material gas, has a light absorption spectrum (ultraviolet wavelength of 130 to 200 nm) of a continuous spectrum at wavelengths of an ultraviolet light of 245 nm or less. By absorbing excimer light of ultraviolet light of 245 nm or less, the oxygen molecule is dissociated into oxygen atoms, and ozone is generated by three-body collision among the oxygen atom resulting from the dissociation, the oxygen molecule, and a third material, which is known in an excimer lamp or the like that emits ultraviolet rays. However, in a silent discharge under high pressure of one atmospheric pressure or higher, which is used in, for example, an ozone generator based on an oxygen gas, there is no emission of excimer light of ultraviolet light of 245 nm or less. Thus, dissociation into oxygen atoms caused by the silent discharge light and a reaction constant during a reaction process in ozone generation are very small. Therefore, it cannot be considered as a reaction that generates a high concentration ozone gas having a high concentration of several % or higher.
Therefore, for supplying ozone to a multiple ozone treatment apparatus, the following ozone gas supply system has been conventionally adopted as disclosed in Patent Document 1, for example. That is, a raw material gas that is a raw material oxygen gas containing a nitrogen gas of several thousands ppm or more, or a raw material gas obtained by forcibly adding a nitrogen gas of several thousands ppm or more to a raw material oxygen gas, is supplied to an ozone generator, to generate high concentration ozone. Additionally, in order to supply an ozone gas to a plurality of ozone treatment apparatuses, the volume of one ozone generator is increased, and a pipe system line that outputs an ozone gas is divided into a plurality of pipes. Thereby, ozone gases each having a predetermined flow rate and a predetermined concentration are stepwisely outputted to a multiple ozone treatment apparatus.
FIG. 28 is a block diagram showing an internal configuration of a conventional ozone gas supply system 70, which can be simulated based on the disclosure of the Patent Document 1.
In FIG. 28, the ozone gas supply system 70 includes one ozone generator 71, one ozone power source 72, one ozone control unit 77, and one system collective management unit 80. The ozone control unit 77 has a raw material gas pipe system line that supplies a raw material gas to the ozone generator 71 via a flow rate controller (MFC) 73a, a flow rate controller (MFC) 73b, and a pressure meter 62. The flow rate controller (MFC) 73a controls the flow rate of a raw material gas that is supplied from a raw material gas supply port 64a. The flow rate controller (MFC) 73b controls the flow rate of a nitrogen gas that is supplied from a nitrogen gas supply port 64b such that the nitrogen gas having a predetermined flow rate is added to the raw material oxygen gas. The pressure meter 62 monitors the pressure of the generator. Additionally, the ozone control unit 77 includes a valve switch 61 and an ozone concentration meter 75. The valve switch 61 adjusts opening/closing of a valve depending on a pressure fluctuation in the ozone generator 71. A part of the output gas pipe system line located subsequent to an output pipe in which an ozone flow meter 67 is provided is divided into a plurality of pipes. Furthermore, in the ozone gas supply system 70, individual ozone gas flow rate controllers (MFC) 68-1 to 68-n are provided to the divided parts of the output gas pipe system line, respectively, so that the ozone gas is independently supplied to a plurality of ozone treatment apparatuses 12-1 to 12-n that are provided corresponding to the individual MFCs 68-1 to 68-n, respectively. An amount of ozone gas exceeding the amount that can be supplied through the individual MFCs 68-1 to 68-n is discharged by a flow rate discharge unit 69.
In an ozone generator disclosed in Patent Document 2, light having a wavelength of the visible light region (visible light of 428 nm to 620 nm) can be emitted (discharged) in a silent discharge caused by an oxygen gas in the ozone generator. A photocatalytic material that absorbs the light having a wavelength of the visible light region which is emitted in the discharge, is applied to a discharge surface of the generator. This causes a raw material oxygen gas to be dissociated due to a photocatalytic effect. A chemical reaction between an oxygen atom resulting from the dissociation and an oxygen molecule of a raw material oxygen generates an ozone gas.